This invention relates to the control of welding power and of welding duration, during the formation of an electrical resistance spot weld, by means of infrared temperature sensing.
In this type of spot weld as illustrated in FIG. 1, two sheets of metal 11 and 12 are joined by melting and fusing a local area 13 under high temperature and pressure. The temperature is achieved by the passage of a momentary high electrical current through the metallic sheets. The current results from the application of a low voltage by a pair of electrodes 14 and 15, usually of copper. The sheets are placed between tips 16 and 17 of the electrodes which are then brought together so as to provide both voltage and pressure.
The pressure, the welding voltage, the welding duration and other variables are under the control of the operator, via a set of panel controls on the welding machine. Once the controls have been adjusted to form a weld with the desired characteristics, they remain undisturbed if it is desired to form a series of identical welds, as in a production operation.
Most usually, however, a series of such welds will show random variations in their properties. Many unacceptable welds will be formed because the current or the pressure was too low. These will be interspersed with both normal welds and overheated welds. The latter are undesirable because they do not meet the standards of strength of appearance, they accelerate the wear of the electrode tips and they are wasteful of electrical energy.
The low-pressure welds result when the sheets are warped or buckled and resist the mechanical force of the electrodes. The low-current and overheated welds result from a variety of reasons:
Supply voltage variation;
Variation in quality and cleanliness of the work sheets;
Progressive deterioration (flattening, corrosion) of the welding tips;
Variations in the temperature or flow rate of the cooling water which circulates within the tips;
Work sheet preheatng due to nearby, previously-formed welds;
Electrical power shunting, or diversion, due to nearby, previously-made welds;
Work sheet "heat-sinking" variations, or differences in the rates at which the work sheets draw heat from the weld, depending on whether the weld is formed near the center of the sheets or near an edge or a corner; or
Any condition which creates high-energy sparks which draw energy from the weld. Sparks can result from low pressures, corrosion or welding at the edge of the work sheets.
It is an accepted fact in the welding industry that these variations do occur. However, it is less practical to try to control all of these variables than it is simply to form an extra number of welds for each pair of work sheets which are to be joined. Thus, the most economical solution is to specify that a certain percentage of additional welds be formed, above the number which would be required if all welds were perfect. The extra amount varies with the welding conditions and the work-quality requirements. A typical range is from 30-100%.
Nonetheless, the extra welds are costly in terms of power, equipment wear, electrode replacement and labor. Several methods have been attempted for providing better control of the spot welding process, but none has met with wide acceptance because they are either costly or ineffective.
In one method, the amount of electrical power applied to each weld is "metered" so that the same amount of energy is expended in each weld, regardless of variations in resistance to the flow of electrical current. Thus, a weld which draws low current is heated for a longer period of time. This method uses the fact that the amount of heat produced depends upon the product of both the current (squared) and the time. Thus, a low current for a long time should provide the same heat as a high current for a short time. However, this is true for only a limited range of current values. One ampere of current for a prolonged time will not produce the same weld as 10,000 amperes for one second. This is because in low-current welds, the heating rate is not sufficient to overcome the cooling influences of the welding tips and the surrounding work sheets. Below a certain current value, the weld can never reach the fusion temperature.
Other weld-quality monitors have been proposed which are used for evaluation purposes but not for the control of the individual weld. They are "after-the-fact" methods which indicate that certain control adjustments should be made. One such method is a "thermal expansion" type in which a sensitive pressure detector monitors the amount and rate of thermal expansion as the weld is being formed. Other methods make use of acoustic emissions which occur during mechanical changes when the weld is being formed. The transmssion of ultrasonic acoustical energy through each weld, during or after formation, is another method of evaluation. As we have indicated, none of these methods is in widespread use.